POSIX Threads Programming
tutorial, at LLNL.
How to Write a Simple Makefile
from "Managing Projects with GNU Make", third edition, by Rober Mecklenburg
The Nachos Instructional Operating System
, by T. Anderson et al.
Xen virtual machine
, developed at the University of Cambridge, England.
The Linux Kernel HOWTO
, by B. Ward.
What really happened on Mars?
, by M. Jones - an extra reason to study operating systems.